1. Field of the Invention
The present invention generally relates to a device for detecting connection or disconnection of a battery to an electric charger.
2. Description of the Relevant Art
Secondary batteries, especially Ni-Cd storage batteries, are used in various fields as power sources for cordless telephones, video recorders and other home electric appliances. Generally, rapid charging is carried out in about 1-2 hours at a charging rate of between 0.5 C and I.5 C. The Ni-Cd cell or battery is frequently used so that during the use thereof, it often requires an electrical charging, and in many cases, the rapid or quick charge is carried out. The quick charging operation means that the battery is subjected to a type of treatment which is carried out under a comparatively severe condition to the battery and therefore, it is necessary for the quick charging to be controlled so as not to overcharge the battery.
A -.DELTA.V system and a temperature control system have been put into practical use as systems for controlling the rapid charge. The temperature control system is carried out in such a manner that a heat sensitive element (such as, a thermistor) is contained in a battery pack, and a temperature detecting line connected to an end of the thermistor is led to a data processor within an electric charger. The thermistor's resistance value is changed depending on a change in the ambient or environmental temperature. Therefore, if a current flowing therethrough or an electric voltage at the end of the thermistor is measured, the temperature around the thermistor can be effectively detected. Accordingly, if the data processor detects that the temperature of the battery has risen at around the time when the charging by the electric charger has ended, the rapid charging operation can be terminated. Thereafter, the rapid charging is switched to an ordinary trickle charging. Trickle charging means a charging operation by which a battery is electrically charged in a continuous manner with a current which is near the self-discharge of the battery.
There is another control system for controlling the rapid charging. This control system operates in such a manner that a timer is provided to control the charging time at a predetermined value. More particularly, the timer is set in advance for preventing the rapid charging from being continued for an excessive period of time on such an occasion when the temperature control system or the -.DELTA.V system fails to operate for some reason, and a further continuous rapid charging is carried out beyond the predetermined time. This timer, however, has to be reset after the operation of the control system has ended or switched to a trickle charging. The timer is usually reset after it is detected that the storage battery after the completion of the charge is removed from the electric charger. Furthermore, the electric charger has lamps for indicating operations (such as, rapid charging and trickle charging), whereby the battery pack is not connected to the battery charger or the like.
The conventional charger is, in general, sufficient in that it detects whether the battery to be charged is connected thereto or not, and the lamp indication is changed over, and the timer is reset. For this purpose, a mechanical switch is used for detecting whether the battery pack is connected to the electric charger or not, and the switch is actuated by the size and weight of the battery. However, if the mechanical switch is used, the reliability thereof is decreased, and it is not suitable for use in a small-sized device. Furthermore, it has been hitherto carried out that the connection or disconnection of the battery pack is detected by a current or a voltage. However, in such an operation, there is the fear of erroneous detection in a case where the battery is inactive or deteriorated.